1. Field of the Invention
This invention relates to a new and novel technology for isolating fermentation salts from aqueous solution. The method of this invention extracts water from the solution to concentrate fermentation salts, in particular, carboxylate salts such as calcium acetate.
2. Review of Related Art
Numerous manufacturing processes produce aqueous wastes or process streams containing carboxylic acid. These include the manufacture of cellulose acetate, aspirin, camphor, and RDX explosives, as well as semi-chemical wood pulping and other processes that use acetic acid as a raw material or solvent. In addition, there are many manufacturing methods for acetic acid involving recovery of acetic acid from aqueous solution. For example, the principal methods to produce acetic acid are carboxylation of methanol, liquid-phase oxidation of hydrocarbons such as butane and oxidation of acetaldehyde. Earlier processes, such as alcohol fermentation and the destructive distillation of wood, yielded dilute aqueous solutions.
More recently, processes have been proposed to produce acetic acid, or other carboxylic acids, from biomass employing rumen microorganisms that produce dilute aqueous solutions. The concentration of acid in these processes must, of necessity, be dilute, because high acid concentrations inhibit microbial growth.
Several processes have been developed to recover acetic acid from aqueous solutions. These methods include liquid-liquid solvent extraction, azeotropic distillation, and extractive distillation. Simple distillation is not appropriate because (1) the relative volatility between water and acetic acid is close to unity and becomes worse for dilute aqueous acetic acid solutions, and (2) water is the more volatile component, compared to acetic acid; this means all the water has to be vaporized from dilute acetic acid solutions, leading to a large energy cost per unit of acetic acid recovered.
Extractive distillation was used for years in the Suida process to recover acetic acid from pyroligneous acid which contains 6-7% acetic acid. Recycled wood oils were used as the extractant. In acetic acid synthesis plants, azeotropic distillation was used for higher concentration streams. Methyl and ethyl acetates, diisopropyl ether, and benzene are commonly used entrainers for azeotropic distillation, although other esters and ethers, ketones, chlorinated hydrocarbons and alcohols have also been used.
In specific applications of acetic acid manufacturing, other processes might be used. For example, freeze concentration has been used for years as a backyard process for concentrating vinegar. Adsorption with carbon or anion exchangers and chemical derivatization, followed by separation and regeneration of the chemical derivatives, are also available. A conventional derivatization method is the calcium acetate process for recovering acetic acid from pyroligneous acid.
In the calcium acetate derivation process, calcium hydroxide reacts with the acid to form calcium acetate, which is concentrated by evaporation. Then a strong acid, such as sulfuric acid, is added to liberate the free acid. This approach consumes chemicals such as lime and sulfuric acid and produces gypsum as a waste aqueous salt by-product.
Recovery of carboxylic acids from water is one of the oldest applications of solvent extraction. Solvent extraction of carboxylic acid was proposed a century ago. With the development of more sophisticated extraction techniques, such as countercurrent extraction, recovery of residual solvent from the raffinate phase by distillation, and the use of azeotropic distillation to remove coextracted water from the recovered acid, solvent extraction of carboxylic acids from aqueous solutions has replaced the calcium acetate derivation process.
In general, extraction from aqueous solutions has heretofore been the most favored approach to recover acetic acid, except for feeds (aqueous solutions) above about 80% (w/w) acetic acid content, where azeotropic distillation is preferred. Based on the development of new extractants, extractive processes, and the change in the economic structure, extraction is now favored for feeds containing acetic acid below 30% (w/w); extractive distillation is preferred for feeds in the 30-80% (w/w) range; and azeotropic distillation and simple fractionation (distillation) are appropriate for more concentrated feeds.
In the literature, solvent extraction has been the main theme for recovering carboxylic acid from dilute aqueous solutions. Because the solvents tend to be partially water soluble, a unit operation is required to remove or recover residual solvent from the raffinate. Typical extraction processes recover carboxylic acid from water using three unit operations: the extractor, a solvent regenerator, and a process to recover free carboxylic acid. In a conventional extraction process, one first extracts acid into the organic phase, followed by back-extracting acid into an aqueous phase, thus regenerating the organic phase. In the regeneration process, the aqueous phase used to back extract acid from the extract is usually an aqueous solution containing a low-boiling alkaline solvent (e.g. trimethylamine) which can be easily evaporated. Using this process, carboxylic acid can be separated and recovered.
Among the extractants used to recover carboxylic acid are reactive, basic extractants (e.g. tertiary amines or phosphine oxides) that can be used to gain greater solvent capacity and selectivity with respect to water and carboxylic acid. Although solvent extraction is a potentially attractive process for carboxylic acid recovery, it is usually deterred by the high affinity of these acids for water. Previous workers have characterized some extractants that provide a relatively high equilibrium distribution coefficient for extracting carboxylic acid from aqueous solution. A high equilibrium distribution coefficient allows the use of lower solvent-to-feed flow rates.
Primary amines are too soluble in water to be used with aqueous solutions. Secondary amines are subject to amide formation upon regeneration by distillation. Consequently, long-chain tertiary amines have become the most favored extractant for recovering carboxylic acid from dilute aqueous solutions. With the use of appropriate diluents and swings of temperature or pH, tertiary amine extractant systems have become more powerful agents for carboxylic acid recovery.
Desalination
Solvent extraction has also been proposed as a method to recover potable water from sea water or brackish water. Extracting sea water with an amine solvent produced a concentrated brine (raffinate) and an extract containing a mixture of water and amine. Clear water was recovered by heating the water-laden extract to a higher temperature at which the solubility of solvent in water was greatly reduced. Previous discussions concerning the solvent extraction of water have been published by Davison et al. (see Davison, et al., "Structure and Amine-Water Solubility in Desalination by Solvent Extraction," J. Chem. Eng. Data 1960, 5, 420-423; Davison, et al., "Thermodynamic Cycles For Recovery of Water by Solvent Extraction," I&EC Process Design and Development, 1964, 3, 399-404; Davison, et al., "Phase Equilibria of Desalination Solvents: Water-NaCl-Amines," J. Chem. Eng. Data. 1966, 2, 304-309; Davison, et al., "A Solvent Extraction Desalination Pilot Plant," Desalination, 1967, 3, 17-26; Davison and Hood, U.S. Pat. No. 3,088,909; and Davison and Hood, U.S. Pat. No. 3,424,675.)
Carboxylic acids from biomass
There has been increasing interest in using anaerobic bacteria to produce organic acids from biomass. A dilute aqueous stream of carboxylic acid can be produced by fermenting lignocellulosic biomass. The concentration in the fermentation broths is limited by the autotoxicity of the acid to acetogenic fermentation bacteria. It may be as high as 6% (w/w), but concentrations of 2-4% are more typical. Product inhibition can be reduced by continuously removing the acid.
Extractive fermentation has been proposed to remove the inhibiting acid in situ and thus increase the bioreactor productivity. However, at the pH where the microbes grow (about 6.8), only 1% is undissociated acid; the rest is salts that will not be extracted, especially by tertiary amines. Most extractants work efficiently only at acidic pH, yet acidogenic anaerobes generally have poor growth rates below pH 6. Thus, extractive fermentation is not viable to remove acid product in situ.
Moreover, after solvent-extracting carboxylic acid from the aqueous fermentation broth using high-molecular-weight tertiary amines, the acid-amine complexes must be broken to recover the free acid and recycle the amine extractant, either by azeotropic distillation or back-titration. For recovering carboxylic acid from dilute aqueous solutions, the extraction-regeneration process requires both acid addition to adjust the pH and an extra solvent-extraction process to regenerate the primary extractant. Furthermore, the tertiary amines extractant (or phosphine oxide extractant) must necessarily have a high molecular weight to make it immiscible in water. However, it is impossible to make these extractant amines perfectly immiscible in water, so some extractant is always present in the aqueous phase. Because of their high molecular weight, these extractants are non-volatile and cannot be recovered by stripping in a distillation column. Therefore, additional extractants must be added to replace that which is lost, thus incurring additional expense.
Necessarily, when produced by fermentation, the acid product is primarily present as a carboxylate salt. The pH of an aqueous solution of carboxylic acids is ordinarily low because the acids dissociate releasing hydronium ions in aqueous solution. To sustain active fermentation, it is necessary to add a neutralizing agent (i.e., lime or limestone) that reacts with the hydronium ion and maintains a pH near to neutrality where microbial activity is greatest. Near neutrality, only about 1% of the product is carboxylic acid; the remainder is present as the carboxylate salt. Therefore the traditional technologies for recovering carboxylic acids are not easily applied, and a need exists for developing of alternative technologies.